We are looking at filling in the back of our lot in the same location that we will run our loop field. Currently the land is heavily wooded and very moist. The average fill over the loop field will be about 8'.

If I trench 6' deep to install my loop field before filling in the lot is this equivalent to burying the pipe at 14'? I guess what I am asking is will the fill, assuming it is clean fill comprising sand, clay and gravel and is properly compacted, require a long amount of time to reach the same temperature as the undisturbed earth?

 

Having a loop buried at 14' will be a very good ground loop heat exchanger

My guess is that it will take about a year for the temperature to become equivalent to the 14' depth.  After going through four seasons the fill should settle more so thus allowing the temperature to permeate the area.  A moist area may equalize even quicker.

Another option is to lay loops on existing grade and fill over the top. If you want to equate to a 14 foot depth, add a few feet of loop. We design our loops to operate at a minimum temperature therefore, more depth equals less feet......
If you do operating cost evaluation, the (percentage) savings is modest for extra large or extra deep fields. The downside to deeper or larger fields is installation/material cost. If these are modest then go for it.
Joe

Thanks for the ideas.

My concern is when you look at a pile of fill in the winter there is often frost on top and the frost penetrates the pile much deeper than the normal frost line (3' here in CT). I am not sure why that happens: compaction, air entrainment, lack of heat conduction from the surrounding earth?

I want to make sure that doesn't happen to the loop field. It would be great to lay the pipes on the existing grade and then fill. For the difference in cost I could raise the grade another 1-2'.